videoscale: Correct DAR and border calculations

changed: gst_video_scale_set_info in gst/videoscale/gstvideoscale.c
DAR on sink side now calculated with PAR on sink side
ratio of output width/height now calculated with inverse PAR
additional condition that borders are 0:0 for passthrough mode
https://bugzilla.gnome.org/show_bug.cgi?id=696019